Wednesday, 10 December 2014

Sharks and Rays of the Eocene Canadian High Arctic.

Palaeontological studies of the Arctic during the Early-to-Middle
Eocene have revealed a world in which the ice-free Arctic Ocean was surrounded
by lush warm-temperate rainforests, inhabited by creatures such as Alligators,
Turtles and Hippo-like Mammals. The fauna of the Arctic Ocean itself is less
well known, however, though Shark’s teeth and Fish otoliths (mineralized
tissues from the ears of Fish and some marine invertebrates) from Ellesmere
Island have been since at least the 1970s, and from Banks Island since the 1980s.

The first Shark taxa recorded is Striatolamia macrota,
a form of Sand Tiger Shark previously recorded from Eocene sediments in a
variety of locations. This is one of the most abundant species present, with
thousands of isolated teeth found.

The second species recorded is an unnamed species of Carcharias (anther Sand Tiger Shark),
referred to as Carcharias sp. A. This
species is also represented by thousands of isolated teeth. Members of the
genus Carcharias are still extant
today, and inhabit warm-temperate and tropical seas.

The third species recorded is a second species of Carcharias, referred to a Carcharias sp. B. Specimens assigned to
species B have week striations, which are absent in species A, and the largest
teeth of species B are notably smaller than the largest teeth of species A.
This species is also represented by thousands of isolated teeth.

The fourth species recorded is Odontaspis winkleri,
a fourth species of Sand Tiger Shark, though in this case only represented by
two isolated teeth. The genus Odontaspis first
appeared in the Late Cretaceous, and is still extant, with a modern species
found in tropical and warm temperate waters in the Atlantic, Indian and Pacific
Oceans and the Mediterranean Sea.

The fifth species recorded is Physogaleus secundus,
an extinct relative of Tiger and Sharpnose Sharks (but not Sand Tiger Sharks,
which are not closely related to Tiger Sharks). This species is recorded from
22 isolated teeth; this being the furthest north the genus Physogaleus has been recorded.

The seventh species recorded is a single tooth also placed in the
genus Physogaleus, but recorded as
cf. Physogaleus americanus; that is to
say Padilla et al. are confident that
the tooth come from a Shark of the genus Physogaleus,
and that they most closely resemble the tooth of Physogaleus americanus, but that they are not completely confident
of this. Physogaleus americanus is
known from the Late Palaeocene and Early Eocene of Mississippi, though the
Banks island tooth is considerably larger than the largest teeth recorded from
Mississippi, at 6.1 mm, compared to an average of 3 mm for the Mississippi
teeth.

The eighth species recorded is a single tooth from an unknown
Myliobatid Ray, Myliobatis sp., the
genus that includes modern Eagle Rays, and which is now found in warm-temperate
to tropical waters across the globe.

The Shark faunal assemblage from the Early Eocene of Banks Island
conforms to the predicted climatic model, being made up entirely of species
found in warm temperate, or warmer, waters today. However it is unusually poor
in species for such an assemblage, with just a tiny handful of specimens that
do not come from the most abundant three species. This is typical of modern
polar species assemblages, which tend to be dominated by one or two abundant
species, but unusual in warmer waters, where a more diverse community is
typical.

There does not seem to be any good reason why a warm Arctic fauna
would be dominated by a low number of species in the way modern cold arctic
faunas are. Instead Padilla et al. propose
an alternative hypothesis for this low species diversity, which is independent
of the latitude of the site. Modern Sand Tiger Sharks often enter brackish (low
salinity) estuarine and delta waters to feed, as to Eagle Rays, but most other
Sharks prefer fully saline waters. The deposits on Banks Island appear
consistent with bar formations around the outer margin of a delta, an
environment where fresh and saline water often mix, producing a low salinity
environment, and Padilla et al. suggest
that this may be the cause of the low species diversity seen there.

The Giant Shark, Carcharocles megalodon,
is one of the more charismatic creatures of the recent fossil record, a
relative of modern Mackerel Sharks that is thought to have been able to reach
about 18 m in length, known from the Middle Miocene to the end of the Pliocene,
with some claims of the species persisting into the Pleistocene. It is
interpreted to have had a life-style similar to the modern Great White Shark,
which preys...

The remains of a variety of non-marine Vertebrates have been recovered
from Ellesmere and Axel Heiberg Islands in the eastern Canadian High
Arctic from the 1970s onwards. These vertebrate assemblages date
from early – middle Eocene (53–50 million years ago), when the islands
were close to their current positions within the...

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About Me

Studied Palaeobiology & Evolution at the University of Portsmouth, Geosciences via the Open University & Ecology and Conservation at Christchurch University, Canterbury.
Have worked in wildlife based tourism, mineral exploration, development, conservation, education & environmental chemistry. Occasionally write articles for papers and magazines.

This Blog would be impossible without the work of countless scientists (and others) throughout the world. Where possible I do my best to credit them, but there will always be many more who remain unmentioned; this does not imply I am ungrateful for their contributions. Any errors or inaccuracies are, of course, my own.